Fix fetching source path of shared files

Fixes #2141 and #2160 

I thought I already fixed the resharing problem, the code was taken directly from apps/files_sharing/public.php I'll be working to consolidate sharing code after OC5.

@icewind1991 I changed lib/files/mount.php because I couldn't find an alternative. Let me know if there is a simpler way to do this. Would I be better off creating a new view for the file owner each time?

Still need to fix the paths returned for the shared status icons. Don't merge yet, just review current code please. 

@schiesbn @karlitschek @MTRichards